The renewed interest in visual persistence or iconic memory, in the last few years has resulted in an extensive literature but little consensus about the phenomenon. Its locus within the visual system, its function on various tasks, and even its relationship with basic stimulus parameters are subject to current debate. The present proposal describes several experiments that seek to clarify the nature of visual persistence by a psychophysical investigation of the phenomenon on classic persistence tasks. The aims of this research are to demonstrate not only the precise effects of several stimulus variables on the degree of visual persistence but also the existence of predictable stimulus interactions among these variables. The predicted pattern of results is based upon a program of research that favors a sensory model of persistence in terms if extended activity at very early--even photoreceptor--levels in the visual system. Moreover, such findings can serve to resolve serious discrepancies among previous studies that have either examined stimulus effects under limited ranges, have ignored the interactive effects of variables, or have inadventantly confounded the different types of persistence by their choice of experimental procedures. The proposed research consists of four psychophysical experiments that examine the effects of target luminance, retinal location, wavelength, and background level of the degree of visual persistence. Furthermore, the comparability of results across two standard persistence tasks (the probe-matching task and the partial-report task) and with very different kinds of stimuli (spatial frequency gratings, pictures, and letters) will be assessed. Each of the four studies focuses on a particular area of controversy in the persistence literature. This research seeks to resolve the current confusion as well as to suggest a basic model of persistence that can incorporate the total range of empirical findings.